Alongside the usual trivia of the silly season - sharks off Cornwall, beasts on Dartmoor, diabetic cats - one August story, which comes round as reliably as airport chaos and a wet bank holiday, is deadly serious for millions of families, as well as for politicians and schools. It concerns the release of GCSE and A-level results. When the national figures for pass rates and grade attainments are published over the next three weeks, they will lead, as they always do, to fierce argument about whether standards are rising or falling. Teachers and government ministers will say they are on the up; opposition politicians and most newspapers will say they are plunging. You will naturally study these arguments carefully, wishing, as a concerned citizen, to know who is right. I can save you the trouble. The argument, as always, will be spurious. It is not possible to compare one year's exam results with those of previous years.

This will seem a startling and even scandalous statement to some, and particularly to journalists who usually have little else to write about in August. But exam results are just sets of figures, and the idea that figures aren't strictly comparable over time is accepted in other areas of life. A million pounds today doesn't represent a fortune, as it did 30 years ago. Governments have changed the basis of unemployment figures at least a dozen times in my memory. The components of the retail price index are reviewed annually. Crime figures are notoriously unreliable, because what counts as a crime changes constantly. The extent of poverty is always contestable, because it is defined in relation to the national average income.

So why do we expect examinations to be immutable? Why, when we are always being told that the world is changing at unprecedented speed, do we want schools to stand still, teaching roughly the same things as 10, 20 or 30 years ago so that we can reliably compare "standards"? Mathematics has been transformed by the advent of pocket calculators and computers. Understanding of the role of DNA has revolutionised biology. Global warming is changing the climate and even the geography of the planet. Language teaching now puts more emphasis on oral fluency, less on written translation. History is no longer a succession of great men and great battles, because no serious scholar now treats it as such.

Many subjects now taken in exams - media studies, environmental studies or information technology, for example - were undreamt of 40 years ago. You can argue that some of these subjects are soft options that should not be offered at A-level or GCSE - though politicians constantly assure us that the environment is the biggest challenge of our age, and no less an authority than the late Anthony Sampson described the growth of media power as the greatest change in Britain over the past 40 years.

But all that is a separate issue. It has no bearing on whether or not standards are falling. If the subject matter varies significantly, the exam will simply be different and therefore comparisons with earlier years will be spurious. The same applies to examining techniques. If coursework assessment, multiple-choice question papers, projects and exams where you can use calculators or reference books are more common than they used to be, the probability (though not certainty) is that more children will get high grades. This is invariably greeted as incontrovertible evidence of lower standards. But why?

The point of introducing a wider variety of examining methods was to give a better chance to those candidates who happened to be weak at writing essays under traditional exam conditions. Other changes - showing the marks available for each question, for example - simply make exams more transparent. No doubt they also make them easier, but there was no merit in keeping candidates in the dark.

Fretting over falling standards is a peculiarly English disease. We believe that a qualification is only worth having if it can be demonstrated that most people would fail it. As Groucho Marx might have said, I don't want to take an exam that would allow me to pass. The schools cannot win either way. If pass rates fall, teachers will be blamed for failing their pupils. If pass rates rise, it will be further evidence of falling standards.

One way of "proving" that the whole exam system has gone to the dogs is to compare the IQs of children attaining particular grades in different years. If those with, say, an IQ of 100 got C this year but only D a few years ago, that is quoted as damning evidence for the prosecution. The possibility that teachers have got better at guiding children through exams is regarded as unthinkable, though another possibility - that exam candidates have benefited from the explosive growth in private, after-school tuition - may be more acceptable to the critics. In any case, IQ scores in industrialised countries rose by about three points per decade from the 1930s onwards. Are we supposed to conclude that the standards of IQ tests have also fallen?

The truth is that to talk about rising or falling standards in the context of school exams is a category error. The exams are not, and have never been, an objective assessment of standards. They are rationing devices. In the days of the 11-plus, pass rates varied capriciously, according to the availability of grammar school places in different parts of the country. We used to pass barely half the entrants to A-levels (against more than 95% now) and those entrants were then drawn from a very small proportion of the population. This was because university places were severely rationed, as were places in such professions as accountancy, medicine and law. The system was designed to produce an elite, and the numbers who could attain each A-level grade were fixed. As RH Tawney put it, thousands of tadpoles died so that a few might become frogs.

It is possible that the young people who now take A-levels are brighter, harder working and better taught than their predecessors. But these things are unknowable. We pass more candidates and give out more A and B grades because we want more people to go to university, judging, rightly or wrongly, that life in a modern economy requires a longer period of initial education. Why should we expect a system that turns a third of the population into graduates to embody the same standards as one that gave degrees to barely one in 20?

A-levels now sort out the top 30% or 40% who should go on to higher education. If we decide that 60% should proceed, the numbers who pass and achieve high grades will rise accordingly. If we decide everybody should go to university, we shall abolish A-levels, as we abolished the 11-plus when we decided everybody should have a decent secondary education. And then what would journalists find to write about in August?